Are you using TypeScript, Babel, etc. or do you have to create code to check if the browser supports fat arrows, template literals, const, etc.? What's the size

Are you using TypeScript, Babel, etc. or do you have to create code to check if the browser supports fat arrows, template literals, const, etc.? What's the size of your team and how many older browsers and old phone browsers do you have to support? If you have to support many browsers, want to use modern language features and you don't want to write code to check that the browser supports modern syntaxes then you do need a build system.